This project is concerned with the behavioral analysis of the biophysical mechanisms underlying cochlear implant function. Behaviorally trained cats will be deafened unilaterally and implanted with one of two multi- contact intracochlear electrode arrays. Two classes of experiments are proposed. The first is to relate psychophysical performance to anatomic findings and several neurophysiological measures (EABRs, intracochlear- evoked potentials, and single-fiber discharge patterns) in response to the same stimuli, all in the same animals. Behavioral tasks include measures of absolute threshold, growth of loudness, dynamic range and channel interactions. In the second class, the goal is to characterize the effects of minute manipulations of intracochlear current fields on psychophysical performance. Using a dense, multicontact linear-array electrode, absolute thresholds for single and paired pulsatile stimuli will be measured as a pulse phase polarity and electrode position. Intracochlear-evoked potentials, EABR and electrical field mapping studies will be undertaken to characterize the growth in electrical field and activation, as a function of stimulus intensity, for the same stimuli as were used behaviorally. At the conclusion of the psychophysical and electrophysiological testing, animals from both classes of experiments will be sacrificed, and their cochleae harvested for histological analyses to determine the number and spatial position of surviving peripheral prncesses and spiral gang lion cells, relative to the electrode contacts. Data from both classes of experiments, combined with predictions of electrical fields from a finite element model, will form inputs into biophysically-based, neural behavioral models. The models will be evaluated in an attempt to account for behavioral performance on the basis of electrical field configuration, the resulting patterns of neural activity, and the anatomic condition of the spiral ganglion in the tested animal. In addition, the feasibility of conducting fine focal stimulation within the cochlea, both within and across a critical band length, will be evaluated in terms of measures of channel interactions and behavioral performance.